Conventional electronic and magnetic ballasts typically use resonant or quasi-resonant circuits to impress an AC current through plasma in a lighting tube of a plasma-based light source. FIG. 1 is a schematic representation of plasma 10 in a tube having one electrode 12 at each end. The plasma is situated between the electrodes and moves back and forth, making contact with one end and then the other. Where contact is made, there is contact resistance Rc. The non-contacted end incurs a small gap in the plasma, which produces a capacitance Cs, through which power must flow. The plasma itself has an ionization potential Vz, a series resistance Rs, a series inductance Lp, and an inertial resistance Ri of the plasma to movement. Additionally, there are thermal effects which need not be considered for purposes of the present disclosure.
An electronic equivalent circuit model of the plasma and tube is shown in FIG. 2. In addition to the parameters discussed above, the equivalent model includes a representation of two other phenomena. A parallel capacitance Cp exists between the lamp terminals T1 and T2 when the lamp is not conducting. This capacitance is a very small value in most tubes. A leakage resistance Rlk can be present on the external surface of the lamp. This resistance may only play a role in areas of very high humidity and dust, dirt or other contaminants. This simplified model of a plasma tube does not consider filaments that may be used in some tubes.
Conventional 60 Hz magnetic ballasts include simple transformers with very high impedance secondary windings to provide an output current that is self-limiting. They also include windings for each of the lamp filaments. At startup the filaments get most of the power, heating up to start ionization.
The secondary voltage builds up to a very high value, and the lamp becomes fully ionized as the gas becomes plasma between the two filaments. The effective resistance of the conducting plasma is quite low and the current flow is limited by the secondary windings' impedance. The transformer core partially saturates, which reduces the power available to the filaments.
The majority of the losses in this circuit come from several areas, which include; core saturation, copper losses in the transformer secondary, and losses in the plasma. The voltage waveform across the electrodes is shown in FIG. 3. The sine wave voltage reaches ionization potential Vz and continues to rise. The current is then limited by the resistance in the ballast secondary windings and is dissipated by the copper losses in the windings. The ionized gas, i.e., plasma, emits photons having wavelengths that are dependent on the chemical makeup of the gas(es) and by the temperature of the plasma that is formed by the ionization of the gas. Excessive current in the plasma causes the plasma temperature to rise until infrared photons are emitted from the plasma. The infrared photons, however, are an inefficient use of energy in lighting applications, since they do not contribute to the visible light output. Instead, the infrared photons give off heat, which cools the plasma. As a result, more energy must be added to the plasma to replace the lost heat, which adds to the inefficiency.
The ‘black body’ radiation (absolute temperature in degrees Kelvin) of the plasma determines the light output. At low plasma temperatures, mostly infrared photons will be generated. At higher plasma temperatures more visible and ultraviolet light than infrared light will be generated. In fluorescent lamps, ultraviolet light is converted to visible light by the phosphor coatings. The spectral radiant emittance is determined by the formula used to determine the black body radiation spectrum:
            U      λ        :=                            8          ·          π          ·                                    (                              k                ·                T                            )                        5                                                (                          h              ·              c                        )                    4                    ·                                    (                                          h                ·                c                                            λ                ·                k                ·                T                                      )                    5                                                    ⅇ                              (                                                      h                    ·                    c                                                        λ                    ·                    k                    ·                    T                                                  )                                      -            1                    ⁢                                                                        ⁢          Where:        ⁢                                Uλ is Spectral Radiant Emittance in W cm−2 μ−1         λ is an integer energy level        T is absolute temperature        h=6.626*10−34 (Planck's constant)        c=2.998*108 (speed of light in a vacuum)        k=1.381*10−23 (Boltzsman's constant)        
As shown in FIG. 4, the shaded area represents excess power of every cycle that must be dissipated in the ballast and lamp(s). This wasted power causes excessive heat in the ballast and lamp(s); which shortens the lifetime of both the ballast and lamp(s).
FIG. 5 illustrates the conduction time for the plasma current, which pulsates due to the off times Toff between the sine wave peaks Ton. This result occurs because the voltage V across the lamp drops below the ionization voltage between pulses Ton, and little or no current I flows during the off time Toff. The plasma begins to cool down during Toff because little or no current I is flowing. The plasma must then be re-heated at the start of the next current pulse. This continual reheating adds to the losses and the heat in the lamp and ballast.